1. The Field of the Invention
This invention relates to apparatus for treatment of a liquid by passage through a semipermeable membrane under pressure, as exemplified by processes such as ultrafiltration and reverse osmosis.
2. The Prior Art
The prior art has developed several types of membrane assemblies employed in apparatus for separation of fluid or liquid constituents. One of the most widely employed is the so-called spiral wound membrane cartridge, such as particularly described in U.S. Pat. Nos. 3,542,203 and 3,417,870. A spiral wound membrane cartridge comprises one or more laminate leaf assemblies of semipermeable membrane sheet, backing sheet and porous grid material. These leaf assemblies are each arranged so that the feed liquid is carried over the membrane surface by the porous grid material while permeate is carried through the backing sheet to a central permeate collection tube. After the leaves are arranged and spirally wound around the central permeate collection tube, a layer of adhesive-carrying, impervious plastic tape is tightly wound around the entire body of the cartridge to hold the wound leaves firmly in place. Before insertion into a pressure-resistant container, seals, most often of the so-called "chevron" type, are fitted onto the outer surface of the cartridge to prevent feed liquid from passing between the cartridge exterior and the pressure-resistant container's inner wall so as to bypass the working passages interior of the cartridge.
The arrangement described above has functioned reasonably well over a number of years. However, larger plants are now being built and operated, and the use of membrane cartridges in a much wider field of use has disclosed several disadvantages. Spacing of the outer circumference of the cartridge from the inner wall of the pressure vessel, in order to accommodate chevron seals, has resulted in a small, but measurable, loss of working volume. This loss has become more significant with employment of larger numbers of larger diameter cartridges. Additionally, dead space between the membrane cartridge outer surface and the pressure-resistant vessel inner wall traps solution which is difficult to remove and clean. This is a serious disadvantage when such cartridges are employed in food product treatment, such as ultrafiltration of whey. Such processes may require relatively frequent cleaning and bactericide treatment to prevent growth of bacteria in such stagnant areas. The chevron seals, while efficient during normal operation, have disadvantages during cleaning. They are essentially one-way devices and prevent the effective use of reverse flow of feed liquid, or back-washing, which is most effective for in-place cleaning. If reverse flow is attempted when chevron seals are employed, they collapse and allow free passage of an inordinate amount of wash liquid around the outside of the cartridge where it is of little consequence. Additionally, the chevron seals themselves, in a large plant, represent an appreciable original and replacement expense. The chevron seal arrangement makes necessary somewhat larger diameter pressure-resistant containers than are required for accommodation of just the cartridges themselves and represent an additional extra construction expense which may be substantial because pressure-resistant containers are generally fabricated of strong and fairly expensive materials.
Recently available commercial membrane cartridge assemblies obviate the need for such seals, permit the use of smaller diameter pressure-resistant containers, and do not create dead space making complete cleaning readily achievable. In such cartridges, the entire circumference of the cartridge is encompassed by the porous grid sheet material and, when installed, this material is in a close, at least partial sealing relationship with the interior surface of the container. In one cartridge assembly, after spiral winding of the membrane, a separate sleeve or sock formed of the porous grid material is slid over the membrane in a snug fit and the assembly is slid into the container. In order for the sock of single-layer porous grid material to fit over the membrane, the sock is softened by placing it in hot water just prior to its receiving the spiral membrane. It will be appreciated that precise sizing of the cartridge assembly is important because if it has too large a diameter, it will be difficult to insert into the container. On the other hand, too small a diameter will result in a loose fit so that there will be no sealing relationship between the sock and the interior surface of the container. For a more complete description of the structure and operation of such cartridge assembly, reference may be made to U.S. Pat. No. 4,301,013.
In another membrane cartridge construction, the membrane is formed by winding two or more leaves with each leaf having a short extension of the porous grid sheet material, with the combined lengths of the extensions being slightly greater than the circumference of the cartridge so that after winding the cartridge is encompassed by the porous grid material. This cartridge is inserted in the container by pushing it in longitudinally while rotating it in a leaf winding direction. Upon release of the cartridge, due to the tendency of the cartridge to unwind, the cartridge will expand slightly placing the outer layer of the porous grid material in intimate sealing contact with the inner surface of the container. Care must be taken in the winding of the leaves because if there is relative slipping, the extensions could overlap with the result that a portion of the circumference of the cartridge may not be encompassed by the porous grid material while other portions have a double thickness of it. Field winding of this cartridge requires care and is time consuming because each leaf must be carefully trimmed to have an extension of proper length. Additionally, the wound cartridge has multiple free ends. In operation, the feed flow could force these free ends downstream, disturbing the flow patterns and causing difficulty in cartridge removal and reinsertion. In an extreme situation, the disruption of flow patterns could cause the membrane surface to enter into contact with the pressure vessel wall thus making it difficult to clean dead or stagnate areas.